1. Field of the Invention
The present invention relates to a magnetostrictive transducer, and a method and apparatus for non-destructive testing of structural defects, and more particularly, to a transducer that generates a shear wave partially on a circumference of a beam, or a pipe having a predetermined cross-section such as a circle and measures and analyses the shear wave which returns after being reflected by a defect, an apparatus for detecting a defect and the position (distance and angle) of the defect in a structure, and a method of operating the transducer.
The present invention was derived from a research project sponsored by the Korea Science and Engineering Foundation and Seoul National University R&DB Foundation.
[2009-0083279, Multi-Scale Paradigm for Creative Design of Multi-Physical Complex Structure System].
2. Description of the Related Art
Magnetostriction refers to mechanical deformation of ferromagnetic materials in a magnetic field. It is also referred as the Joule effect. An inverse effect thereof is referred as an inverse magnetostrictive effect or the Villari effect in which a magnetic state of a material changes when stress is applied thereto. Magnetostriction may be used to measure the deformation of an object without mechanically contacting the object, and is thus widely employed in various fields where contact type sensors cannot be used. In the case of magnetostriction, an elastic wave may be generated contactlessly, and furthermore, a larger elastic guided wave may be generated compared to the case when the conventional piezoelectric effect is used. Examples of inductive ultrasonic waves that may be generated in a waveguide such as a rod or a pipe are a longitudinal wave, a flexural wave, and a torsional wave. For example, in a first mode of the torsional wave, dispersion, which results in speed differences of the torsional wave due to frequency components, does not exist, and thus a structural defect may be effectively diagnosed. Also, a shear wave may be generated partially on the structure, and since a first mode of the shear wave also has non-dispersion characteristics, like the torsional wave, the shear wave may also be efficiently used in monitoring defects.
FIGS. 1 through 3 illustrate a torsional vibration in a rod member when magnetostriction is used and the principle of measuring the torsional vibration.
Referring to FIG. 1, when a first magnetic field BS and a second magnetic field BD are applied perpendicularly to each other near a ferromagnetic strip 1, the strip 1 may be deformed in a shearing direction as illustrated in FIG. 2. That is, the first magnetic field BS is a static magnetic field, which is maintained constant, and the second magnetic field BD is a dynamic magnetic field. The strip 1 is deformed according to the variation of the second magnetic field BD as illustrated in FIG. 2.
Referring to FIG. 3, the above principle is explained in relation to a rod member. The strip 1 is attached along the circumference of a rod member 2. Then, a static magnetic field BS is applied around the strip 1, and a variable dynamic magnetic field BD is applied in a length direction of the rod member 2 that substantially perpendicularly crosses the static magnetic field BS. Thus, a torsional vibration may be generated in the rod member 2 according to the deformation of the strip 1.
FIGS. 4 and 5 schematically illustrate the configurations of conventional magnetostrictive transducers for generating a torsional wave.
Referring to FIG. 4, the conventional magnetostrictive transducer that generates a torsional wave includes a thin ferromagnetic strip 1 which is wound around a rod member 2 in the circumferential direction of the rod member 2, an insulator 4 that is installed on the circumference of the strip 1, and a solenoid coil 3 that is wound around the insulator 4. In order to generate a torsional wave in the magnetostrictive transducer having the above-described configuration, the ferromagnetic strip 1 need to be rubbed using a magnet in order to pre-magnetize the ferromagnetic strip 1.
However, the ferromagnetic strip 1 may not be uniformly pre-magnetized, and thus, measurement repetitiveness may not be ensured.
To avoid pre-magnetizing the ferromagnetic strip 1, a method of forming a magnetic field in a ferromagnetic strip 1′ in a desired direction by further disposing a magnet 6 in the magnetostrictive transducer of FIG. 4 as illustrated in FIG. 5 has been suggested.
However, in this case, although a structure may be monitored by generating a torsional wave, only a distance between a measuring unit and a position of a structural crack may be measured, and additional information such as an angular position along a circumferential direction in a waveguide may not be obtained.